THE PRESENT INVENTION relates to an electric control circuit for a vehicle safety device, and more particularly relates to an electric control circuit for a vehicle safety device which may be activated by passing current through an electric resistor or squib.
An electric resistor or squib is often used, in the field of vehicle safety devices, as an initiator for a pyrotechnic charge. The pyrotechnic charge may form the operative part of a gas generator adapted to generate gas to inflate an air-bag, or may form part of a pre-tensioner, that is to say a device adapted to apply tension to a safety belt in the event that an accident should occur.
It has been proposed to connect the electric resistor that forms the initiator for the pyrotechnic charge in a series connection which includes two switches. Both of the switches must be closed before current can pass through the electric resistor. Typically, the switches are located one on either side of the electric resistor so that, when the switches are open, the electric resistor is totally isolated from the rest of the circuit. This minimises the risk of any current inadvertently passing through the electric resistor, which might activate the vehicle safety device at an inappropriate moment.
It has been proposed previously to provide two sensors, each sensor being adapted to close one of the two switches.
One of the sensors, which may be called a "safing sensor" or "crash risk sensor" is adapted to sense a parameter which indicates that the vehicle in which the safety device is fitted is exposed to a risk that a crash might occur. Typically, the safing sensor incorporates a spring-biassed mass that will respond to a medium level of deceleration. To ensure that the switch that is controlled by the safing sensor does not open in response to a short deceleration, as might occur during an accident, typically a pulse generator is provided which is associated with the safety sensor and which provides an extended pulse having an extended duration of, for example, 220 ms which ensures that the switch is maintained closed during this period of time. The crash risk sensor could also comprise a radar or some other device adapted to determine the speed of the vehicle relative to, the distance of the vehicle from, and/or the direction of an external object.
The purpose of the switch controlled by the safing sensor is to arm the arrangement when there is a risk that a crash may occur. At other times, the arrangement is not armed and thus the electric resistor is totally isolated from the rest of the circuit.
The other sensor, a crash sensor, may be an accelerometer associated with a microprocessor which is adapted to determine when the signal generated by the accelerometer is indicative that an accident is occurring.
Typically, the current is supplied, to one of the switches, from a capacitor which is charged by a charging circuit. If both the switches are closed, the entire charge of the capacitor passes through the resistor, thus causing the resistor to heat to a high temperature enabling the resistor to initiate the pyrotechnic charge.
However, with a prior-proposed arrangement of this type, some difficulties can arise.
Typically the switches are formed of MOSFET devices. If the MOSFET switch associated with the crash sensor suffers from "leakage", if the switch associated with the safety sensor is closed, for example, if the vehicle is driving along a bumpy road, the charge from the capacitor may leak away relatively slowly, without the pyrotechnic charge being activated. Should an accident then occur, there may be insufficient charge remaining on the capacitance in order to trigger the pyrotechnic charge.
Additionally, if, for example, there should be a failure of the MOSFET switch associated with the crash sensor, in such a way that the switch is conductive (or if there is a short circuit between the resistor and the battery) then as soon as the arming sensor closes the arming switch, the pyrotechnic charge will be ignited.